Compact electronic devices such as portable terminal devices, widely spread in recent years, have been required to be smaller and lighter and have longer life. Accordingly, batteries as power sources for such devices, specifically, secondary batteries capable of producing a high energy density with a small size and a light weight, have been developed. Recently, the application of secondary batteries has been studied not only for compact electronic devices but also for large electronic devices such as cars.
Secondary batteries based on various charge-discharge principles have been proposed. Among them, attention has focused on aluminum secondary batteries based on aluminum deposition-dissolution reaction. This is because aluminum has a high ionization tendency and thus can produce a large quantity of electricity per unit volume through oxidation-reduction reaction. In other words, aluminum is a highly promising material for forming electrodes or as a charge carrier.
An aluminum secondary battery includes a positive electrode, a negative electrode, and an electrolyte including an aluminum salt and a solvent. The composition of the electrolyte, which functions as a medium for charge-discharge reaction, has a great effect on battery performance. Therefore, a variety of studies have been performed on the electrolyte.
For example, alkyl sulfone such as dimethylsulfone is used in the electrolyte in order to achieve high capacity and long life (see, for example, Patent Document 1). In this case, an organic solvent such as a cyclic or chain carbonate or a cyclic or chain ether is used.
In order to reduce polarization during discharge, an aluminum salt such as aluminum chloride and an organic halide such as trimethylphenylammonium chloride are used together in the electrolyte (see, for example, Patent Documents 2 and 3). In this case, an organic solvent such as 1,2-dichloroethane is used.
To put aluminum secondary batteries to practical use, it is necessary to allow aluminum deposition-dissolution reaction to proceed smoothly and sufficiently even at relatively low temperature (e.g., room temperature). In conventional aluminum secondary batteries, however, aluminum deposition-dissolution reaction can proceed smoothly and sufficiently only at relatively high temperature (e.g., 50° C. or higher), and the efficiency of the reaction is also low, which has room for improvement in terms of practicality.
Thus, the applicant has previously proposed that an electrolyte for use in an aluminum secondary battery should include an aluminum salt, alkyl sulfone, and a solvent with a dielectric constant of 20 or less so that the electrolyte can reduce the temperature at which aluminum deposition-dissolution reaction proceeds (see Patent Document 4).